DocID10153 Rev 2 7/14
TSM1013 Principle of operation and application hints
14
6 Principle of operation and application hints
6.1 Voltage and current control
6.1.1 Voltage control
The voltage loop is controlled via a first transconductance operational amplifier, the resistor
bridge R
1
, R
2
, and the optocoupler which is directly connected to the output.
The relation between the values of the R
1
and R
2
should be chosen as written in Equation 1.
Equation 1
R
1
= R
2
x V
ref
/ (V
out
- V
ref
)
Where V
out
is the desired output voltage.
To avoid the discharge of the load, the resistor bridge R
1
, R
2
should be highly resistive. For
this type of application, a total value of 100 K (or more) would be appropriate for the
resistors R
1
and R
2
.
As an example, with R
2
= 100 K , V
out
= 4.10 V, V
ref
= 2.5 V, then R
1
= 41.9 K.
Note that if the low drop diode should be inserted between the load and the voltage
regulation resistor bridge to avoid current flowing from the load through the resistor bridge,
this drop should be taken into account in Equation 1 by replacing V
out
by (V
out
+ V
drop
).
6.1.2 Current control
The current loop is controlled via the second transconductance operational amplifier, the
sense resistor R
sense
, and the optocoupler.
The V
sense
threshold is achieved externally by a resistor bridge tied to the V
ref
voltage
reference. Its middle point is tied to the positive input of the current control operational
amplifier, and its foot is to be connected to the lower potential point of the sense resistor as
shown in Figure 4. The resistors of this bridge are matched to provide the best precision
possible.
The control equation verifies:
Equation 2
R
sense
x I
lim
= V
sense
V
sense
= R
5
x V
ref
/ (R
4
+ R
5
)
Equation 3
I
lim
= R
5
x V
ref
/ (R
4
+ R
5
) x R
sense
where I
lim
is the desired limited current, and V
sense
is the threshold voltage for the current
control loop.
Note that the R
sense
resistor should be chosen taking into account the maximum dissipation
(P
lim
) through it during the full load operation.
